U.S. patent number 9,230,571 [Application Number 14/468,399] was granted by the patent office on 2016-01-05 for mgo based perpendicular spin polarizer in microwave assisted magnetic recording (mamr) applications.
This patent grant is currently assigned to Headway Technologies, Inc.. The grantee listed for this patent is Headway Technologies, Inc.. Invention is credited to Wenyu Chen, Yan Wu.
United States Patent |
9,230,571 |
Chen , et al. |
January 5, 2016 |
MgO based perpendicular spin polarizer in microwave assisted
magnetic recording (MAMR) applications
Abstract
A design for a microwave assisted magnetic recording device is
disclosed wherein a spin torque oscillator (STO) between a main
pole and write shield has a spin polarization (SP) layer less than
30 Angstroms thick and perpendicular magnetic anisotropy (PMA)
induced by an interface with one or two metal oxide layers. Back
scattered spin polarized current from an oscillation layer is used
to stabilize SP layer magnetization. One or both of the metal oxide
layers may be replaced by a confining current pathway (CCP)
structure. In one embodiment, the SP layer is omitted and spin
polarized current is generated by a main pole/metal oxide
interface. A direct current or pulsed current bias is applied
across the STO. Rf current may also be injected into the STO to
reduce critical current density. A write gap of 25 nm or less is
achieved while maintaining good STO performance.
Inventors: |
Chen; Wenyu (San Jose, CA),
Wu; Yan (Cupertino, CA) |
Applicant: |
Name |
City |
State |
Country |
Type |
Headway Technologies, Inc. |
Milpitas |
CA |
US |
|
|
Assignee: |
Headway Technologies, Inc.
(Milpitas, CA)
|
Family
ID: |
54939260 |
Appl.
No.: |
14/468,399 |
Filed: |
August 26, 2014 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G11B
5/147 (20130101); G11B 5/314 (20130101); G11B
5/3146 (20130101); G11B 5/315 (20130101); G11B
5/1278 (20130101); G11B 5/3909 (20130101); G11B
2005/0024 (20130101) |
Current International
Class: |
G11B
5/127 (20060101); G11B 5/31 (20060101); G11B
5/39 (20060101); G11B 5/00 (20060101) |
Field of
Search: |
;360/125.3,128 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
"Microwave Assisted Magnetic Recording," by Jian-Gang Zhu, et al.,
IEEE Transactions on Magnetics, vol. 44, No. 1, Jan. 2008, pp.
125-131. cited by applicant .
"Spin-Torque Oscillator Based on Magnetic Tunnel Junction with a
Perpendicularly Magnetized Free Layer and In-Plane Magnetized
Polarizer," by Hitoshi Kubota, et al., 2013 The Japan Society of
Applied Physics, Applied Physics Express 6 (2013) 103003, Sep. 27,
2013, pp. 1-3. cited by applicant .
"High-Power Coherent Microwave Emission from Magnetic Tunnel
Junction Nano-oscillators with Perpendicular Anisotropy," by
Zhongming Zeng, et al, 2012 American Chemical Society, Jun. 4,
2012, vol. 6, No. 7, pp. 6115-6121. cited by applicant.
|
Primary Examiner: Klimowicz; Will J
Attorney, Agent or Firm: Saile Ackerman LLC Ackerman;
Stephen B.
Claims
We claim:
1. A microwave assisted magnetic recording (MAMR) structure that
includes a write head, comprising: (a) a main pole that generates a
magnetic flux field which is directed through a pole tip at an air
bearing surface (ABS) and into a magnetic medium to write one or
more bits, the magnetic flux has a gap field component that is
directed across a spin torque oscillator (STO) and into a write
shield; (b) the write shield with a side along the ABS that
collects magnetic flux which has passed through the magnetic medium
and written the one or more bits; (c) an external current source
that produces a direct current bias or pulsed current bias between
the main pole and write shield, and across the STO; and (d) the STO
that is formed along the ABS and generates a if field on the
magnetic medium and thereby assists the writing to one or more
bits, the STO comprises; (1) a spin polarization (SP) layer which
is selected from CoFeB, CoFe, CoFeNi, CoB, or FeB with
perpendicular magnetic anisotropy (PMA) that spin polarizes the
bias current in a direction perpendicular to a top surface of the
SP layer and towards an oscillation layer (OL); (2) the oscillation
layer (OL) wherein the spin polarized bias current of a critical
current density from the SP layer causes magnetization in the OL to
oscillate with a sufficiently large angle and frequency to generate
the if field on the magnetic medium to assist the writing to one or
more bits; (3) a non-magnetic spacer between the SP layer and OL,
the non-magnetic spacer comprises one or more metal oxides; (4) a
seed layer formed between the main pole and the SP layer, the seed
layer is a metal oxide layer consisting of AlOx, TaOx, or RuOx, a
laminate of one or more of AlOx, TaOx, and RuOx, or a laminate of
MgO with one or more of AlOx, TaOx, and RuOx; and (5) a capping
layer formed between the OL and the write shield wherein the seed
layer and the non-magnetic spacer induce the PMA in the SP
layer.
2. The MAMR structure of claim 1 wherein the SP layer has a
thickness less than 30 Angstroms.
3. The MAMR structure of claim 2 wherein the PMA in the SP layer
may be increased by decreasing a thickness of the SP layer.
4. The MAMR structure of claim 1 wherein back scattered spin
polarized current from the OL works with a damping torque to
stabilize the SP layer against undesired magnetization switching in
the SP layer.
5. The MAMR structure of claim 1 wherein a thickness of the STO in
a down-track direction between the main pole and write shield
represents a write gap (WG) distance, and the WG distance is about
25 nm or less.
6. The MAMR structure of claim 1 wherein the seed layer has a
thickness from 10 to 20 Angstroms.
7. The MAMR structure of claim 1 wherein the non-magnetic spacer is
a metal oxide layer or a laminate that is comprised of one or more
of AlOx, MgO, AlTiOx, MgZnOx, and ZnOx.
8. The MAMR structure of claim 1 wherein the non-magnetic spacer
has a thickness from 10 to 20 Angstroms.
9. The MAMR structure of claim 1 wherein the OL is comprised of a
Co alloy, a Fe alloy, or is a laminate with an (A1/A2).sub.n stack
of layers where n is a lamination number, A1 is one of Co, Fe,
CoFe, CoFeR in which R is a non-magnetic element, and A2 is one of
Ni, NiCo, and NiFe.
10. The MAMR structure of claim 1 wherein a rf current bias may be
combined with the direct current bias or the pulsed current bias
across the STO to reduce the critical current density.
11. A method to form a microwave assisted magnetic recording (MAMR)
write head, comprising: (a) providing a main pole that generates
magnetic flux which is directed through a main pole tip at an air
bearing surface (ABS) and into a magnetic medium comprised of a
plurality of bits, the magnetic flux has a gap field component that
is directed across a spin torque oscillator (STO) and into a write
shield; (b) forming the STO on the main pole, the STO comprises:
(1) a seed layer that contacts a surface of the main pole that
faces a down-track direction, the seed layer is a metal oxide layer
consisting of AlOx, TaOx, or RuOx, a laminate of one or more of
AlOx, TaOx, and RuOx, or a laminate of MgO with one or more of
AlOx, TaOx, and RuOx; (2) a spin polarization (SP) layer which is
selected from CoFeB, CoFe, CoFeNi, CoB, or FeB having a bottom
surface contacting the seed layer, the SP layer has perpendicular
magnetic anisotropy (PMA) and spin polarizes a bias current from an
external current source in a direction perpendicular to a top
surface of the SP layer and towards an oscillation layer (OL); (3)
a non-magnetic spacer that contacts the top surface of the SP layer
and a bottom surface of the OL, the non-magnetic spacer comprises
one or more metal oxides; (4) the oscillation layer (OL) wherein
the spin polarized current from the SP layer has a critical current
density that causes magnetization in the OL to oscillate with a
sufficiently large angle and frequency to generate a rf field on
the magnetic medium to assist the writing to one or more bits; and
(5) a capping layer formed on a top surface of the OL, and wherein
the seed layer and the non-magnetic spacer induce the PMA in the SP
layer; (d) forming the write shield on the capping layer; and (e)
connecting the external current source by a lead to the main pole
and with a lead to the write shield.
12. The method of claim 11 wherein the SP layer is has a thickness
less than about 30 Angstroms.
13. The method of claim 12 wherein the PMA in the SP layer may be
increased by decreasing a thickness of the SP layer.
14. The method of claim 11 wherein a thickness of the STO in the
down-track direction between the main pole and the write shield
represents a write gap (WG) distance, and the WG distance is about
25 nm or less.
15. The method of claim 11 wherein the seed layer has a thickness
from 10 to 20 Angstroms.
16. The method of claim 11 wherein the non-magnetic spacer is a
metal oxide layer or a laminate that is comprised of one or more of
AlOx, MgO, AlTiOx, MgZnOx, and ZnOx.
17. The method of claim 11 wherein the OL is comprised of a Co
alloy, a Fe alloy, or is a laminate with an (A1/A2).sub.n stack of
layers where n is a lamination number, A1 is one of Co, Fe, CoFe,
CoFeR in which R is a non-magnetic element, and A2 is one of Ni,
NiCo, and NiFe.
Description
RELATED PATENT APPLICATIONS
This patent application is related to U.S. Pat. No. 7,978,442 and
U.S. Pat. No. 8,582,240, both assigned to a common assignee, and
herein incorporated by reference in their entirety.
TECHNICAL FIELD
The present disclosure relates to shrinking the write gap distance
between a main pole and write shield in a MAMR design for
perpendicular magnetic recording (PMR) applications, and in
particular to modifying a spin torque oscillator (STO) structure
wherein a spin polarizer (SP) layer with perpendicular magnetic
anisotropy (PMA) is thinned by including one or two adjoining metal
oxide layers that enhance PMA in the SP layer while maintaining STO
performance.
BACKGROUND
As the data areal density in hard disk drive (HDD) writing
increases, write heads and media bits are both required to be made
in smaller sizes. However, as the write head size shrinks, its
writability degrades. To improve writability, new technology is
being developed that assists writing to a media bit. Two main
approaches currently being investigated are thermally assisted
magnetic recording (TAMR) and microwave assisted magnetic recording
(MAMR). The latter is described by J-G. Zhu et al. in "Microwave
Assisted Magnetic Recording", IEEE Trans. Magn., vol. 44, pp.
125-131 (2008).
Spin transfer (spin torque) devices in MAMR writers are based on a
spin-transfer effect that arises from the spin dependent electron
transport properties of ferromagnetic-spacer-ferromagnetic
multilayers. When a spin-polarized current passes through a
magnetic multilayer in a CPP (current perpendicular to plane)
configuration, the spin angular moment of electrons incident on a
ferromagnetic layer interacts with magnetic moments of the
ferromagnetic layer near the interface between the ferromagnetic
and non-magnetic spacer. Through this interaction, the electrons
transfer a portion of their angular momentum to the ferromagnetic
layer. As a result, spin-polarized current can switch the
magnetization direction of the ferromagnetic layer, or drive the
magnetization into stable dynamics, if the current density is
sufficiently high.
Referring to FIG. 1, a generic MAMR writer based on perpendicular
magnetic recording (PMR) is depicted. There is a main pole 1 with a
sufficiently large local magnetic field to write the media bit 5 on
magnetic medium 4. Magnetic flux 8 in the main pole proceeds
through the air bearing surface (ABS) 6-6 and into medium bit layer
4 and soft underlayer (SUL) 7. A portion of the flux 8a returns to
the write head where it is collected by write shield 2. For a
typical MAMR writer, the magnetic field generated by the main pole
1 itself is not strong enough to flip the magnetization of the
medium bit in order to accomplish the write process. However,
writing becomes possible when assisted by a spin torque oscillator
(STO) 3 positioned between the main pole and write shield 2. The
STO and medium bit 5 are enlarged in FIG. 1 side (b) and the former
is comprised of a high moment magnetic layer 10, and a second
magnetic layer 11 called a spin polarization (SP) layer that
preferably has perpendicular magnetic anisotropy (PMA). Between
layers 2 and 10, 10 and 11, and 11 and 1, there are nonmagnetic
layers 12, 13, 14, respectively, made of a metal to prevent strong
magnetic coupling between adjacent magnetic layers.
Assuming a medium bit 5 with a magnetization in the direction of 9
(pointing up) is being written by a flux field 8 pointing down as
in FIG. 1 side (a), part of the magnetic flux 8b goes across the
gap between main pole 1 and write shield 2, and this weak magnetic
field can align the magnetization of SP layer 11 perpendicular to
the film surface from left to right. An external current source 18
creates a bias current I across the main pole and write shield. The
applied dc results in a current flow in a direction from the write
shield through the STO 3 and into main pole 1.
Referring to FIG. 2a, the direct current generated by source 18 is
spin polarized by magnetic layer 11, interacts with magnetic layer
10, and produces a spin transfer torque .tau..sub.s 23 on layer 10.
Spin transfer torque has a value of a.sub.j
m.times.m.times.m.sub.p, where a.sub.j is a parameter proportional
to the current density j, m is the unit vector 15 in the direction
of the instantaneous magnetization for layer 10, and m.sub.p is the
unit vector 16 in the direction of magnetization in layer 11. Spin
transfer torque .tau..sub.s 23 has a representation similar to the
damping torque .tau..sub.D 24, and with a specific current
direction, .tau..sub.s 23 competes with .tau..sub.D 24, so that the
precession angle 50 is from about 0 to 10 degrees. Only when the
current density is above a critical value j.sub.c will .tau..sub.s
23 be large enough to widely open the precession angle of
magnetization 15 in layer 10 such that the oscillation has a large
angle 51 usually between 60.degree. and 160.degree. as indicated in
FIG. 2b. The large angle oscillatory magnetization of layer 10
generates a radio frequency (rf) usually with a magnitude of
several to tens of GHz. This rf field 49 (FIG. 1 side b) interacts
with the magnetization 9 of medium bit 5 and makes magnetization 9
oscillate into a precessional state 17 thereby reducing the
coercive field of medium bit 5 so that it can be switched by the
main pole field 8.
The magnetic layer 10 is referred to as the oscillation layer (OL),
and the aforementioned oscillation state is also achieved if main
pole field 8 and medium magnetization 9 are in the opposite
directions to those shown in FIG. 1. In this case, the direction of
the SP magnetization 16 will be reversed, and OL as well as the
medium bit will precess in the opposite direction with respect to
the illustration in FIG. 1 side b.
In the prior art, seed layer 14 is typically thicker than 3 nm to
prevent SP layer to main pole (MP) coupling. Moreover, SP layer 11
is normally >8 nm in thickness since laminates such as
(Co/Pt).sub.n, (Co/Ni).sub.n, and (FeCo/Ni).sub.n where n is a
lamination number of about 5 to 30 are used to establish strong
PMA. Total PMA for the SP layer must overcome the perpendicular
demagnetization field so that SP magnetization 16 stays in the
perpendicular to film plane direction without an external field.
Non-magnetic layer 13 is typically 2 nm thick and the thickness of
OL 10 is generally >10 nm. If the OL is too thin, magnetic
moment 15 of OL cannot deliver a large enough rf field to assist
recording. Non-magnetic layer 12 (often called a capping layer in
the bottom STO design in FIG. 1b) is preferably >5 nm thick.
Thus, a conventional STO stack representing the write gap is at
least 28 nm thick. For a stable STO that can function with good
yield, another 5-10 nm may need to be allocated to SP 11, OL 10,
and capping layer 12 for a total STO thickness well over 30 nm.
Z. Zeng et al. in "High-Power Coherent Microwave Emission from
Magnetic Tunnel Junction Nano-oscillators with Perpendicular
Anisotropy", ACS Nano Vol. 6, No. 7, pp. 6115-6121 (2012), and H.
Kubota et al. in "Spin-Torque Oscillator Based on Magnetic Tunnel
Junction with a Perpendicularly Polarized Free Layer and in-Plane
Magnetized Polarizer" in Applied Physics Express (Jap. Soc. of App.
Physics), 6, pp. 103003-1-103003-3 (2013) both present ideas for
devices such as an rf signal source and rf field detector with high
power narrow band rf emission using a magnetic tunnel junction
(MTJ) having a magnetic layer with PMA. A common feature is a MTJ
that is comprised of a free magnetic layer with PMA that is not
based on a laminated structure such as (Co/Ni).sub.n, but rather
PMA is established by an interface between a MgO layer and a thin
CoFeB or FeB layer. As a result, the free layer can be more easily
driven into large angle oscillation with high power and a narrow
frequency band. However, the aforementioned MTJ structure cannot be
applied in a STO for MAMR applications mainly because the thin free
layer has a low magnetic moment due to its thickness of around 2 nm
that is required for the induced anisotropy field to overcome its
demagnetization field in the perpendicular to plane direction. Even
if the free layer can be driven into an oscillation angle at a
similar level to that depicted in FIG. 1, total rf field applied to
the magnetic medium is only 20% of that produced by a conventional
STO for MAMR because of the low magnetic moment.
Clearly, current STO technology must be improved to enable a
smaller write gap (WG) without compromising the rf field strength
directed onto the magnetic medium in a MAMR application. PMR writer
production already requires a WG to be at 25 nm, significantly less
than state of the art STO structures with a WG near 30 nm or
larger, in order to achieve a large enough field gradient for good
writing performance. Although MAMR improves writing performance, a
larger WG than desired reduces the field inside the head between
the MP and write shield that is needed for MAMR to work at the
required frequency. Therefore, reducing the WG thickness in the STO
stack to 25 nm or less becomes a necessity if MAMR is to be widely
accepted in the industry.
SUMMARY
One objective of the present disclosure is to reduce the thickness
of a STO stack formed between a main pole and write shield in a
MAMR writer thereby shrinking the write gap for PMR
applications.
A second objective of the present disclosure is provide a thinned
STO stack according to the first objective while maintaining the
effectiveness of the SP and OL layers to deliver a rf field of
sufficient strength to assist the write process.
A third objective of the present disclosure is to provide a thinned
STO stack according to the first objective by using existing
materials and processes that do not complicate STO fabrication.
According to one embodiment of the present disclosure, these
objectives are achieved with a MAMR design having a STO stack of
layers formed between a main pole and a write shield, and wherein
the leads from the main pole and write shield enable a current to
pass through the STO stack. The STO stack may have a bottom spin
valve configuration wherein a seed layer, SP layer, non-magnetic
spacer, OL, and capping layer are sequentially formed on the main
pole and the capping layer contacts the write shield. SP layer
thickness is less than 30 Angstroms (3 nm) and preferably from
about 6 to 20 Angstroms so that substantial PMA is established when
one or both of a SP layer top surface and bottom surface contact a
metal oxide layer. Preferably, one or both of the seed layer and
non-magnetic spacer are made of a metal oxide such as MgO to
generate sufficient PMA to overcome the perpendicular
demagnetization field within the SP layer and enable the SP layer
to spin polarize the current directed to the OL. In another
embodiment, the STO has a top spin valve configuration wherein a
seed layer, the OL, non-magnetic spacer, SP layer, and capping
layer are sequentially formed on the main pole. In this case, one
or both of the non-magnetic spacer and capping layer are a metal
oxide to enable a thin SP layer with substantial PMA.
A direct current or pulsed current flowing through the STO stack
from the main pole to the write shield is converted to spin
polarized current by the SP layer and interacts with the OL to
cause the latter to oscillate with a large angle and a frequency
that generates a rf field on a nearby magnetic medium thereby
assisting a magnetic field from the main pole to switch a magnetic
bit during a write process.
According to another embodiment, one or both of the seed layer and
non-magnetic spacer in the STO bottom spin valve configuration has
a confined current pathway (CCP) structure wherein metal pathways
such as Cu are formed in a metal oxide matrix. In a STO top spin
valve configuration, one or both of the non-magnetic spacer and
capping layer may have a CCP structure in order to establish
substantial PMA in the adjoining SP layer and improve reliability
of the STO.
In yet another embodiment, the SP layer and non-magnetic spacer may
be omitted and the main pole/metal oxide seed layer interface
serves as a source of spin polarized current. Here, a seed layer
that is a metal oxide, the OL, and a capping layer are sequentially
formed on the main pole. The main pole/metal oxide interface
generates PMA in a portion of the main pole adjoining the seed
layer such that current entering the STO is polarized in the
perpendicular to plane direction and causes oscillation of
sufficient magnitude within the OL to generate a rf field that
assists the write process on the magnetic medium.
In all embodiments, a current is applied between the main pole and
write shield during the write process and may be a direct current
or a pulsed current. In some embodiments, a rf current injection
may be combined with the direct current or pulsed current to reduce
critical current density in the STO.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross-sectional view of a conventional MAMR structure
with a STO formed between a main pole and write shield wherein an
external direct current source creates a bias current between the
main pole and write shield.
FIG. 2a is a cross-sectional view of the spin polarization (SP)
layer and oscillation layer (OL) in the STO in FIG. 1 and depicts a
small precession angle in the OL magnetization.
FIG. 2b is a cross-sectional view of the SP layer and OL in FIG. 1
when the critical current density in the STO is above a threshold
value that causes the OL magnetization to achieve a large
precession angle and produce a rf field.
FIG. 3 is a cross-sectional view of a magnetic layer wherein
perpendicular magnetic anisotropy is generated by an interface with
one or both of a top non-magnetic layer and a bottom non-magnetic
layer comprised of a metal oxide.
FIG. 4 is a down-track cross-sectional view of a MAMR structure
according to an embodiment of the present disclosure wherein a STO
is formed between a main pole and a write shield in a bottom spin
valve configuration, and has at least one metal oxide layer
adjoining a spin polarization layer.
FIG. 5 is a down-track cross-sectional view of a MAMR structure
according to an embodiment of the present disclosure wherein a STO
is formed between a main pole and a write shield in a top spin
valve configuration, and has at least one metal oxide layer
adjoining a spin polarization layer.
FIG. 6 is a modification of the MAMR structure in FIG. 5 wherein
the STO has a confined current pathway (CCP) layer between a SP
layer and an oscillation layer.
FIG. 7 is another embodiment of the present disclosure wherein the
STO in FIG. 6 is modified to include a CCP layer as the capping
layer in the STO stack.
FIG. 8 is another embodiment of the present disclosure wherein the
STO in FIG. 7 is modified to replace the CCP layer between the SP
layer and OL with a non-magnetic spacer that is a metal oxide or a
metal.
FIG. 9 is a modification of the MAMR structure in FIG. 4 wherein
the STO has a confined current pathway (CCP) layer between a SP
layer and an oscillation layer.
FIG. 10 is another embodiment of the present disclosure wherein the
STO in FIG. 9 is modified to include a CCP layer as the seed layer
in the STO stack.
FIG. 11 is another embodiment of the present disclosure wherein the
STO in FIG. 10 is modified to replace the CCP layer between the SP
layer and OL with a non-magnetic spacer that is a metal oxide or a
metal.
FIG. 12 is a down-track cross-sectional view of a MAMR structure
according to another embodiment of the present disclosure wherein a
metal oxide seed layer, oscillation layer, and capping layer are
sequentially formed on a main pole layer.
FIG. 13 is another embodiment of the present disclosure wherein the
STO in FIG. 12 is modified to replace the seed layer with a CCP
structure between the main pole and the OL.
FIG. 14 is another embodiment of the present disclosure wherein a
combination of a direct or pulsed current, and a rf current are
injected into the main pole layer to enable OL frequency to be
tuned and to reduce critical current density in the STO.
FIG. 15 is a down-track cross-sectional view of a portion of a PMR
write head including a STO formed between a main pole and write
shield.
DETAILED DESCRIPTION
The present disclosure is a MAMR structure comprising a spin torque
oscillator (STO) formed between a main pole and a write shield in a
PMR write head, and in particular, relates to a SP layer with
reduced thickness that is enabled by an SP layer interface with one
or two metal oxide layers. Thus, the write gap distance between
main pole and write shield is reduced while maintaining STO
performance in assisting the magnetic pole during a write process.
In all drawings, the z-axis is a down-track direction, the y-axis
is a cross-track direction, and the x-axis is along a direction
between an air bearing surface and a back end of the MAMR
structure.
Referring to FIG. 3, a magnetic layer 26 is formed between two
non-magnetic layers 25, 27 and may be a reference layer or free
layer in a magnetic tunnel junction, for example. According to one
aspect, when the magnetic layer is a laminate such as (Co/Ni).sub.n
where n is a lamination number, perpendicular magnetic anisotropy
PMA 29 is intrinsic because of the spin-orbit interactions between
the 3d and 4s electrons of Co and Ni atoms. When considering a
candidate for a thinned SP layer in a STO stack that is useful for
WG reduction, a laminate is not acceptable because a laminate
thickness of at least 8 nm is necessary to provide sufficient PMA.
We have found that a magnetic layer with a thickness in the range
of 2 to 3 nm is ideally suited as a thin SP layer wherein PMA is
induced by at least one metal oxide/SP layer interface. Thus, when
magnetic layer 26 is a thin Co alloy or Fe alloy about 6 to 30
Angstroms thick, and one or both of non-magnetic layers 25, 27 are
MgO or another metal oxide, there is sufficient PMA 29 generated by
interfaces 28a, 28b for the magnetic layer to serve as a SP layer
in a STO device.
The total interface induced perpendicular anisotropy energy
potential (E.sub.ani) for a magnetic layer is expressed by the
equation E.sub.ani=-nAS where A is an interface property related
constant, S is the interface area S, and n is the number of
interfaces (1 or 2) for the magnetic layer with a metal oxide
layer. The effective anisotropic field is expressed by
H.sub.ani=-.differential.E.sub.ani/.differential.m=nA/(M.sub.st),
where m is the total magnetic moment of the magnetic layer, M.sub.s
is the magnetization density, and t is the thickness of the
magnetic layer. The H.sub.ani equation indicates that increasing t
causes a weaker perpendicular anisotropy field, and conversely, a
stronger perpendicular magnetic anisotropy (PMA) is produced by
reducing the thickness t.
One important requirement of a SP layer is that the perpendicular
anisotropy field (H.sub.ani) must be large enough to overcome the
demagnetization field in the perpendicular direction in the absence
of an external magnetic field. Conventional SP layers meet this
requirement by way of a thickness that is generally 8 nm or
greater. The demagnetization field is expressed by
H.sub.demag=4.pi.M.sub.SN.sub.Z where N.sub.Z is the geometric
demagnetization factor in the perpendicular (z-axis) direction, and
typically N.sub.Z.ltoreq.1 for a thin magnetic film. Note that the
demagnetization is a bulk parameter and only N.sub.Z is weakly
dependent on the layer thickness if t is substantially less than
the width of the magnetic layer in a cross-track direction. Thus,
once the magnetic layer thickness t satisfies the equation
t<nA/4.pi.Ms^2Nz, the magnetic moment of the magnetic layer can
be aligned in a direction that is perpendicular to the film plane.
Typically, the magnetic layer thickness must be thinner than 3 nm
to achieve PMA in the absence of an external field when there is no
intrinsic PMA property available by way of a laminated stack in the
prior art.
Although PMA is one requirement for a SP layer in a STO device, it
is not the only desired property in a SP layer. Another necessary
property is sufficient coercive field (Hc) in a perpendicular to
plane direction such that SP layer magnetization cannot be easily
flipped. Hc may be expressed as Hc=H.sub.ani-H.sub.demag. Moreover,
Hc should not be so large that a gap field 8b (FIG. 1) of several
thousand Oersted is not able to switch the magnetic layer's
magnetization. Based on the H.sub.ani equation, one is able to tune
both PMA and Hc by adjusting the thickness of the magnetic layer,
and the number of metal oxide/magnetic layer interfaces (one or
two).
A third important property of a SP layer is stability in the
context of spin transfer torque induced by back-scattered spin
currents. It is known that a SP layer 11 in FIG. 1, for example,
generates a spin polarized current to interact with OL 10.
Furthermore, OL 10 may also spin polarize the current to interact
with SP layer 11. Therefore, a SP layer must be stable to back
scattered spin current from an oscillation layer. We will discuss
this aspect in more detail later with regard to the embodiments of
the present disclosure.
In FIG. 4, a first embodiment of the present disclosure is
illustrated and retains the main pole, write shield, and magnetic
medium structures of FIG. 1. In this case, a direct current (100%
duty cycle) or pulsed current I flows from a source 18 through lead
36 to main pole 1 and then passes through STO 40 and write shield 2
before exiting through lead 37. The pulsed current may be at a
scale of 0.1 ns "on" followed by an off period of a fraction of a
nanosecond to multiple nanoseconds. A key feature is STO 40 that
has a bottom spin valve configuration wherein a seed layer 41, SP
layer 42, non-magnetic spacer 43, OL 44, and capping layer 45 are
sequentially formed on the main pole such that a bottom surface of
the seed layer contacts the main pole and a top surface of the
capping layer contacts the write shield, and one or both of layers
41, 43 are a metal oxide to induce PMA in the SP layer less than 30
Angstroms thick. As mentioned previously, the z-axis is the medium
moving direction and is also known as the down-track direction.
During a write process, magnetic flux 8 passes through the ABS 6-6
and transits the magnetic medium 4 and soft underlayer 7 and flux
8a re-enters the write head through the write shield 2. Under a gap
field 8b of several thousand Oe and a dc bias across the STO, the
write process is assisted by a spin polarized current (not shown)
passing from the SP layer 42 to the OL 44 with sufficient magnitude
(critical current density) to cause a large angle oscillation 47
with a certain amplitude and frequency in the OL that imparts a rf
field 49 on medium bit 5. The combined effect of the rf field and
magnetic field 8 enables the magnetization 9 in the bit to be
switched with a lower magnetic field than when only magnetic field
8 is applied. The advantage of the present disclosure is that
critical current density of the spin polarized current may be
lowered to 10.sup.7 A/cm.sup.2 compared with a value of 10.sup.8
A/cm.sup.2 in conventional STO designs because of the higher spin
polarization induced by at least one SP layer/metal oxide interface
as described below with regard to seed layer 41 and non-magnetic
spacer 43. As a result, concern for STO reliability is
substantially minimized.
According to one aspect, the seed layer 41 is a non-magnetic layer
that may a typical seed layer comprised of one or more metals or
alloys such as Ta, Ru, NiCr, and the like, or the seed layer may be
a metal oxide layer. When the seed layer is a metal oxide or a
laminate made of one or more of MgO, AlOx, TaOx, or RuOx,
interfacial perpendicular anisotropy is induced in SP layer 42. The
metal oxide or laminated may be sputter deposited or formed by any
oxidation process of one or more metal layers, and preferably has a
thickness of about 10 to 20 Angstroms.
SP layer 42 may be comprised of a Co alloy or a Fe alloy including
but not limited to CoFeB, CoFe, CoFeNi, FeB, and CoB, and has a
thickness of <30 Angstroms, and preferably 20 Angstroms or less
to enable PMA therein to be maximized according to the H.sub.ani
equation presented earlier. The aforementioned thicknesses
represent a considerable decrease from the usual 80 Angstrom or
larger thickness for a conventional SP layer, and offer a realistic
approach to achieve a write gap of 25 nm or less. It is believed
that Hc will be sufficiently large to prevent the magnetic moment
of the SP layer from being too easily flipped. As for stability
with regard to back scattered spin current from the OL, we believe
that under the bias direction where positive current flows from the
SP layer to the OL, the OL oscillates with the correct chirality to
assist recording, and the back scattered spin current works with
the damping torque to stabilize the SP layer magnetization. In
other words, although the SP layer may be vulnerable to spin
transfer torque due to its low moment, the SP layer is still
expected to be stable. It should be understood that with an
opposite current polarity, the dynamics induced by the spin
transfer torque will be quite significant. However, the opposite
current polarity is not a factor since it does not help MAMR nor is
the opposite current polarity required by a MAMR write process.
Non-magnetic spacer 43 may be a metal layer with good conductivity
such as Cu. In an alternative embodiment, the non-magnetic spacer
is comprised of a metal oxide layer or laminate that is one or more
of AlOx, MgO, AlTiOx, MgZnOx, and ZnOx. TaOx and RuOx are not good
choices for the non-magnetic spacer since they are spin sinks and
will not allow enough spin polarized current to reach the OL to
cause oscillation at a desired frequency and large angle. The
non-magnetic spacer has a thickness of about 30 Angstroms when made
of a metal, and preferably 10 to 20 Angstroms when comprised of a
metal oxide.
OL 44 may be comprised of Fe alloys or Co alloys such as CoFeB,
CoFe, CoFeNi, or a combination thereof, or may be a laminated stack
(A1/A2).sub.n where n is a lamination number and A1 is one of Co,
Fe, CoFe, CoFeR in which R is a non-magnetic element, and A2 is one
of Ni, NiCo, and NiFe, although other magnetic materials are
acceptable. Magnetization of the OL may be in-plane or
perpendicular to the plane of the layer in the absence of a bias
current.
Capping layer 45 is a non-magnetic layer that is preferably one or
more of Ta, Ru, and Cu, although other conductive materials that
may also function as an etch mask during formation of a STO pattern
are acceptable. STO layers mentioned above have planes that are
aligned in the (x, y) plane and have a thickness in a z-axis
direction. Preferably, one or both of seed layer 41 and
non-magnetic spacer 43 are a metal oxide layer in order for PMA
(H.sub.ani) in the SP layer 42 to be sufficiently large so that
spin polarized current flowing to the OL 44 is capable of causing a
large angle oscillation in the OL at a frequency closely matching
that of the bit to be written to in the magnetic medium.
Referring to FIG. 5, another embodiment of the present disclosure
is shown wherein the order of forming layers on the main pole 1 is
seed layer 51, OL 52, non-magnetic spacer 53, SP layer 54, and
capping layer 55 to yield a STO 50 with a top spin valve
configuration. The OL 52 and SP layer 54 have a composition and
properties similar to OL 44 and SP layer 42, respectively.
Likewise, non-magnetic spacer 53 has a composition like that of
non-magnetic spacer 43 in the first embodiment. Seed layer 51 is a
non-magnetic layer that is one or more of Ta, Ru, NiCr, or other
metals or alloys employed as a seed layer in the art. Capping layer
is single layer or composite comprised of one or more of Ta, Ru,
and Cu, or may be a metal oxide layer that is made of one or more
of MgO, AlOx, TaOx, or RuOx. As in the bottom STO embodiment, there
are one or two metal oxide layers that contact the SP layer and
enable the SP layer to have a thickness in the range of 20 to 30
Angstroms to significantly shrink the WG distance while generating
substantial PMA and sufficient Hc to perform adequately as a spin
polarization layer. In this case, one or both of non-magnetic
spacer 53 and capping layer 55 may be a metal oxide layer. Direct
or pulsed current polarity direction from source 18 will be in the
opposite direction to that shown in FIG. 4 in order for the current
to be spin polarized by the SP layer before acting on the OL.
Referring to another embodiment of the present disclosure as
depicted in FIGS. 6-8, STO 50 with a top spin valve configuration
in FIG. 5 may be modified such that one or both of the non-magnetic
spacer 53 and capping layer 55 have a confining current pathway
(CCP) structure wherein metal pathways made of Cu or the like are
formed in a metal oxide matrix. We have previously disclosed a CCP
structure and a method of making the same in U.S. Pat. No.
7,978,442. The metal oxide layer may be comprised of Al, AlCu, Mg,
MgCu, Ti, Cr, Zr, Ta, Hf, Fe, or the like. A pre-ion treatment
(PIT) and ion-assisted oxidation step are used to convert a
conductive metal into conductive pathways formed in the metal oxide
matrix. This method may be employed to improve the conductivity and
reliability of a STO stack compared with a STO described in FIG. 5
where a metal oxide layer is used as one or both of the
non-magnetic spacer and capping layer on either side of the SP
layer. It is understood that the magnitude of E.sub.ani is not as
great as when a uniform metal oxide is employed for the SP
layer/metal oxide interface. H.sub.ani will be reduced somewhat in
proportion to the metal pathway content in the CCP layer.
Referring to FIG. 6, the STO structure is retained from FIG. 5
except that non-magnetic spacer 53 is replaced by a CCP layer 63
wherein conductive metal pathways 61 are formed within a metal
oxide matrix 62. Note that the conductive pathways in STO 60a are
formed substantially in a perpendicular to plane direction. In one
aspect, the metal pathways may comprise about 10% of the CCP layer.
However, the conductive metal content in the CCP layer may vary
depending on the CCP layer thickness and the conditions during the
PIT and IAO process steps. Preferably, when a CCP layer 63 is
employed between the SP layer and OL, the capping layer 55 is a
metal oxide layer to induce PMA in the SP layer.
In FIG. 7, the embodiment shown in FIG. 6 is further modified to
include a second CCP layer 65 as a replacement for capping layer 55
to form STO 60b. Thus, CCP layer 65 has conductive pathways 64
formed within a metal oxide matrix 66. It should be understood that
the conductive pathways in the second CCP layer may be formed of a
different metal than the conductive pathways within CCP layer 63.
Likewise, the metal or alloy selected for metal oxide matrix 66 may
differ from the metal or alloy used to make metal oxide matrix
62.
Referring to FIG. 8, the present disclosure also encompasses an
embodiment wherein the STO structure shown in FIG. 5 is modified to
provide a STO structure 60c. All layers are retained from STO 50
except the capping layer which is replaced by a CCP layer 65 as
previously described. Preferably, when a CCP layer is employed as
the capping layer, the non-magnetic spacer 53 is a metal oxide
layer in order to induce PMA in the SP layer. In all STO
embodiments with atop spin valve configuration, PMA 56 in SP layer
54 is advantageously used to spin polarize a current that causes OL
52 to have a large angle oscillation 57 for the purpose of
producing a rf field 49 on the medium bit 5. Furthermore, the write
gap may be reduced to 25 nm or less in view of the substantial
decrease in SP layer thickness from .gtoreq.8 nm to less than 3
nm.
The present disclosure also anticipates that the STO in FIG. 4 may
be modified to replace one or both of the seed layer 41 and
non-magnetic spacer 43 with a CCP layer as illustrated by the
embodiments found in FIGS. 9-11.
Referring to FIG. 9, the STO structure is retained from FIG. 4
except that non-magnetic spacer 43 is replaced by a CCP layer 73
wherein conductive metal pathways 71 are formed within a metal
oxide matrix 72. The conductive pathways in STO 70a are formed
substantially in a perpendicular to plane direction. The thickness
and conductive metal content in the CCP layer may vary depending on
the conditions during the PIT and IAO process steps used to form
the CCP layer. Preferably, when CCP layer 73 is used between the SP
layer and OL, the seed layer 41 is a metal oxide layer in order to
induce PMA in SP layer 42.
In FIG. 10, the embodiment shown in FIG. 9 is further modified to
include a second CCP layer 75 as a replacement for seed layer 41 to
form STO 70b. Thus, CCP layer 75 has conductive pathways 76 formed
within a metal oxide matrix 74. Conductive pathways in the second
CCP layer 75 may be formed of a different metal than in the
conductive pathways within CCP layer 73. Moreover, the metal or
alloy selected for metal oxide matrix 72 may differ from the metal
or alloy used to make metal oxide matrix 74.
Referring to FIG. 11, the present disclosure also encompasses an
embodiment wherein the STO shown in FIG. 4 is modified to provide a
STO structure 70c. All layers are retained from STO 40 except the
seed layer is replaced by a CCP layer 75 as previously described.
Preferably, when a CCP layer is used as the seed layer, the
non-magnetic spacer 43 is a metal oxide layer so that PMA is
induced in SP layer 42. In all STO embodiments with a bottom spin
valve configuration, PMA 46 in SP layer 42 is advantageously used
to spin polarize a current i that causes OL 44 to have a large
angle oscillation 47 in order to generate a rf field 49 on the
medium bit 5 that assists the write process. Moreover, a
substantial reduction in the write gap to 25 nm or less is achieved
because of a reduction in SP layer thickness from 8 nm or greater
to less than 3 nm.
Referring to FIG. 12, another embodiment of the present disclosure
is depicted wherein the MAMR structure from FIG. 4 is retained
except the SP layer and interlayer are omitted so that a seed layer
41 having a metal oxide composition, OL 44, and capping layer 45
are sequentially formed on the main pole 1 to yield STO 80. Here,
the main pole/metal oxide interface 1s generates PMA in a portion
of the main pole proximate to the seed layer. As a result, direct
current or pulsed current flowing through the main pole from lead
36 is spin polarized in a perpendicular to plane direction
(perpendicular to interface 1s) and interacts with the OL to cause
a large angle oscillation 47 therein with a frequency to produce a
rf field 49 that assists the write process involving magnetization
9 in bit 5.
In an alternative embodiment illustrated in FIG. 13, the metal
oxide layer that serves as the seed layer in FIG. 12 may be
modified to have a CCP structure 83 with confining current pathways
81 formed in a metal oxide matrix 82 as disclosed previously in
U.S. Pat. No. 7,978,442. In effect, CCP structure 83 serves as a
seed layer for uniform growth in overlying layers OL 44 and capping
layer 45 in STO 80a. A CCP seed layer is expected to provide
improved reliability for the STO device compared with an embodiment
in FIG. 12 where the seed layer 41 is a conductive metal oxide
layer with no metal pathways therein.
Referring to FIG. 14, another embodiment of the present disclosure
involves the addition of a bias T 90 in the circuit between main
pole 1 and write shield 2 such that rf current may be combined with
dc or pulsed current to give a current .sub.2 that is injected into
STO 80. Rf current with a frequency f from 0.1 to 50 GHz is
produced by a rf current generator 93. Direct current or pulsed
current source 18 is connected by lead 37 to write shield 2 and is
also connected to the bias T 90 by lead 95. The dc terminal of the
bias T is an inductor 91 with a typical inductance of 0.1 to 10
milli-Henry. For any current including dc with a frequency below
the kHz regime, the impedance of the inductor is smaller than 100
Ohm so that dc and low frequency signal can pass. However, for a
current with a frequency in the GHz regime, the impedance of the
inductor 21 is greater than sub-MegaOhm which blocks the rf
signal.
The bias T 90 has a rf terminal 92 that is a capacitor, preferably
in the 1 nanofarad to 500 nanofarad regime. The rf terminal is
connected to rf generator 93 by a lead 96. Thus, GHz frequency may
pass with low impedance while low frequency current including dc is
blocked with high impedance. In this way, both dc and rf current
are injected from the bias T into STO 80 through lead 36 and main
pole 1. One terminal of the rf generator may be connected to ground
94. However, the main pole and write shield are electrically
floating.
When dc (or pulsed current) and rf current are simultaneously
injected into STO 80 in a current perpendicular to plane (CPP)
mode, the dc and rf current are spin polarized by main pole 1 and
generate a spin transfer torque on OL 44 thereby leading to a large
angle oscillation 47 therein with a certain amplitude and frequency
that produces a rf field 49. We have disclosed in U.S. Pat. No.
8,582,240 that the frequency f1 at which OL 44 naturally oscillates
may be tuned to a value f1' when the rf current frequency f equals
the resonance frequency f0 for magnetization 9 in medium bit 5.
Furthermore, the injection of rf current may be advantageously used
for one or more beneficial effects including OL magnetization
frequency locking, frequency pulling and mixing, and reduced
critical current. The rms amplitude of rf current is preferably
between 0.2 and 5.times.10.sup.8 A/cm.sup.2. The simultaneous
injection of rf current with dc or pulsed current is another means
of reducing direct or pulsed current density from around 10.sup.8
A/cm.sup.2 to about 10.sup.7 A/cm.sup.2. As a result, STO
reliability is now in a safer regime with a lesser concern about
electromigration and interlayer diffusion that are typically
associated with a high current bias in conventional MAMR
technology.
It should be understood that the approach of simultaneously
injecting a dc or pulsed current in combination with rf current
into the STO may also be applied to the embodiments shown in FIGS.
4-11 to reduce the critical current density and thereby improve
device stability.
The present disclosure also encompasses a microwave assisted
magnetic recording (MAMR) write head and a method of making the
same on a substrate that may comprise a read head structure in a
PMR head with a combined write head/read head configuration.
Referring to FIG. 15, a main pole 1 is provided with a side along a
plane 6'-6' that becomes the eventual air bearing surface (ABS)
following a back end lapping process. The main pole has a top
surface 1t facing a down-track direction. Next, a STO structure 40
is formed on the top surface of the main pole such that a side of
the STO is at the ABS, and all layers in the STO stack have a top
surface formed parallel to the main pole top surface, and wherein
the plane of each layer is aligned orthogonal to the ABS.
According to the exemplary embodiment, the STO stack has a seed
layer/SP layer/non-magnetic spacer/OL/capping layer configuration
as represented by layers 41-45 described previously. In an
alternative embodiment (FIG. 5), the STO stack has a top spin valve
configuration wherein layers 51, 52, 53, 54, and 55 described in a
previous embodiment are sequentially formed on the main pole. It
should also be understood that one or more of the layers 41, 43 may
be replaced by CCP layers 75, 73, respectively, in a bottom spin
valve embodiment (FIGS. 9-11), and that one or more of the layers
53 and 55 may be replaced by a CCP layer 63, 65, respectively, in a
top spin valve embodiment (FIGS. 6-8).
Returning to FIG. 15, the STO stack of layers is patterned by a
conventional photolithography and etch sequence to form a sidewall
40s that is parallel to the ABS and is separated therefrom by a
distance w. An insulation layer 100 may be formed on portions of
the main pole not covered by the patterned STO. Then, the write
shield 2 is deposited on a top surface of the insulation layer and
on the capping layer 45. An external current source 18 is connected
to the main pole and to the write shield by leads 36, 37.
Thereafter, the write head is completed by using conventional
methods to form additional layers on the write shield, and a
lapping process is performed to generate an ABS 6-6 (FIGS. 4-11)
that is proximate to the plane 6'-6' shown in FIG. 15.
The embodiments of the present invention provide an advantage over
conventional MAMR designs in several aspects. First, the
incorporation of one or two metal oxide layers along top and bottom
surfaces of the SP layer enable PMA therein and allow a
significantly thinner SP layer than in the prior art. The one or
two metal oxide layers are also typically thinner than a
conventional non-magnetic spacer with the result that total STO
thickness (equivalent to write gap spacing between main pole and
write shield) is reduced by at least 6-8 nm to enable better areal
density capability. Accordingly, the write gap target of 25 nm or
less for current and future generations of MAMR devices is
satisfied while maintaining other STO properties. STO reliability
may be optimized by a combination of one or more modifications
including (a) pulsed current rather than direct current injection
into the STO; (b) replacing one or both metal oxide non-magnetic
spacers adjoining the SP layer with a CCP structure; (c) omitting
the SP layer and one non-magnetic spacer such that the main
pole/seed layer interface generates polarized current; and (d)
simultaneous injection of dc or pulsed current and rf current to
enable a smaller critical current density in the STO device. The
STO devices disclosed herein may be fabricated with standard
materials and methods without any additional steps to complicate
the manufacturing process flow.
While the present disclosure has been particularly shown and
described with reference to, the preferred embodiment thereof, it
will be understood by those skilled in the art that various changes
in form and details may be made without departing from the spirit
and scope of this disclosure.
* * * * *